The present invention relates to the manufacture of articles from sinterable powders by forming processes such as injection molding, extrusion, or the like. More particularly, the invention relates to thermoplastic binder formulations for the manufacture of ceramic or other inorganic products, which binders exhibit reversible gelling behavior in combination with desirable binder burnout characteristics.
Binders useful for the manufacture of products from powdered starting materials, e.g., from particulate ceramic materials, must meet a number of requirements. For example, the binder must be compatible with the ceramic material such that a flowable dispersion comprising a relatively high loading of the ceramic material in the binder may be provided. In addition, the "green" preform produced by shaping the dispersion of ceramic powder in the binder should have reasonable strength such that it can be handled.
For desirable binder "burnout" or dewaxing performance, the binder should be removable from the shaped ceramic part without incurring distortion or breakage of the part. And, the binder-free preform should have at least a minimum level of strength, yet be sufficiently free of binder residues that defect-free consolidation is readily achievable.
The formulation of binders meeting these requirements is complex and a large number of different binder formulations have been proposed in the prior art. U.S. Pat. No. 4,201,594, for example, discloses binders of liquid type which are useful to make slurries of ceramic powders which will gel in a forming mold. The gels thus provided are irreversible, and are intended to improve shape retention in the formed part.
Thermoplastic or heat-softening binder compositions comprising combinations of thermoplastic polymer and oil or wax additives are also known, these compositions taking the form of meltable solids. U.S. Pat. No. 4,568,502 discloses binder formulations of this type, the disclosed compositions consisting of mixtures of thermoplastic block copolymers, oils, waxes, and a stiffening polymer component which enhances the rigidity of green ceramic preforms containing the binder.
U.S. Pat. No. 4,404,166 discloses a binder system comprising two wax components, each component exhibiting a different melting or vaporization behavior. Advantageously, the lower melting point wax can be volatilized from a green ceramic preform comprising the binder while the higher melting point component remains in place to support the preform and help to prevent distortion or sagging thereof.
U.S. Pat. Nos. 4,158,688 and 4,158,689 describe binder formulations comprising block copolymers of specified composition as the principal binder components and added oils and/or waxes to serve as plasticizers. The block copolymers may comprise elastomeric materials, i.e., synthetic rubber components, but the plasticizing oil and wax additives are identified as naphthenic, paraffinic or mixed naphthenic/paraffinic oils or waxes.
While the above and other prior art patents show significant advances in the capability of the art to form complex ceramic products by casting, molding, extrusion or other forming processes, further improvements in this capability are always desirable.
Particularly advantageous would be a binder formulation imparting the characteristic of extensibility to the compounded batch material, i.e., the capability of being extended or stretched without fractural failure of the plastic material. Forming processes requiring extensibility of the batch material during forming are known, but such processes have not been traditionally used for forming ceramic compositions because the required properties were felt to be unattainable with the highly filled compositions generally employed.
Also desirable would be binder compositions exhibiting improved dewaxing or debindering behavior, such that the removal of waxes or other binder constituents from the shaped part could be carried out rapidly without risk of slumping or cracking of the formed part. Particular advantages are realized if lower melt viscosities and lower dewaxing temperatures can be employed.
It is therefore a principal object of the present invention to provide novel binder formulations for powder forming processes and particularly for the manufacture of ceramic parts or products from powdered ceramic materials.
It is a further object of the invention to provide novel thermoplastic binder formulations which can be used to formulate ceramic batches exhibiting sufficient extensibility, at semi-molten binder temperatures, to be compatible with thermo-forming, blow molding and other plastic forming processes.
It is a further object of the invention to provide thermoplastic binder formulations for powder forming which impart sufficient extensibility to the batch to permit cross-sectional area reductions by drawdown of the shaped batch, as for example by draw-down of the extrudate in an extrusion process.
It is a further object of the invention to provide novel thermoplastic binder formulations which can be used to make powder batches comprising sinterable inorganic powders and offering a rapid transition on cooling to a tack-free state, even while retaining molten wax constituents therein.
It is a further object of the invention to provide novel thermoplastic binder formulations which provide green ceramic or other powder-formed parts offering improved dewaxing behavior with reduced part distortion or slumping.
It is a further object of the invention to provide novel ceramic or other powder batches of high solids content and improved methods for compounding them.
It is a further object of the invention to provide novel ceramic batches for the manufacture of ceramic parts or products, which batches exhibit improved batch flow and dewaxing behavior.
Other objects and advantages of the invention will become apparent from the following description thereof.